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National Advisory Committee for Aeronautics, Technical Notes - Ionization in the Knock Zone of an Internal Combustion Engine

The ionization in the knock zone of an internal—
combustion engine was investigated. K suspected correla—
tion between the intensity of knock and the degree of
ionization was verified and-an oscillation in the degree
of ionization corresponding in frequency to the knock
vibrations in the cylinder pressure was observed.

When knock occurs in an internal—combustion engine,
it is accompanied by increased pressures and temperatures
in the cylinders owing to the rapid burning of the end
gas. The violent chemical reaction and the accompanying
increased temperatures of the gases in the knock zone sug—
gest that the ionization of the gases is increased during
the knocking cycle and further, that this increase might
be used to indicate the presence and the intensity of
knock.

Ionization gaps have been used to measure velocity of
flame travel by both Schnauffer and Schutz (references 1
to 3), but no published material seems to be available for
the simple measurement of the degree of ionization in the
knock region of an internal— combustion engine.

In order to investigate the degree of ionization in
the knock zone of an internal—combustion engine, simulta~
neous records of cylinder pressure and ionization were
obtained. A high-speed G.F.R. single-cylinder test engine
was used. The cylinder—pressure indicator consisted of a
quartz crystal pick-up unit, an amplifier. and a 9-inch
cathode~ray oscilloscope. (See references 4 to 6.) The
cylinder~pressure indicator was used to indicate knock.

A 5-inch oscilloscope (reference 7) was mounted di~
rectly above the 9—inch tube so that the two could be pho-
totraphed simultaneously. The current passing between the
Points of a BG-332 spark plug placed in the knock zone
controlled the vertical deflection of the cathode beam of
the 5~inch oscilloscope and served as a measure of ioniz—
ation.

The synchronizer unit (reference 8) contains two al~
ternators that are directly connected to the crankshaft.
One alternator generates six cycles per revolution of the
crankshaft and is normally used for synchronization but
was used, in these tests, as a marker. The marker voltage
is supplied to both oscilloscopes by turning on the marker
amplifier and closing switch SQ (fig. 1). Thus points
are located on the traces of both oscilloscopes correspond—
ing to 60° angles of the crankshaft. From these records
(see fi%. 2) the horizontal time scale of the oscilloscopes
can be calculated. The second alternator. which generates
two cycles per revolution. was used for synchronization.
The two alternators were interchanged to give a.greater
number of marker cycles on the records.